One common type of air-cooled heat exchanger includes a tube bundle having a large number of thin-walled, copper parallel tubes connected in pairs at their ends by return bends to form a fluid circuit. Thin metal, parallel plates, referred to as fins, are secured generally transverse to the tubes to transfer heat from the tubes. These tube bundles in turn must be supported by additional structure. One common practice is to provide a rigid connection between the tubes of the tube bundle and two or more spaced support plates or other support means at the end of the tubes and sometimes in center portions of the tubes.
When the heat exchanger is a refrigeration condenser, air is passed over the tubes of the condenser in order to lower the temperature of, and hence condense, vapor refrigerant flowing through the tubes from a refrigerant compressor. During this cooling process, the tube bundle is subject to vibrations caused by pulsations of the fluid flowing within the condenser. Also, motors and fans moving the cooling air produce vibrations. In addition, the tubes forming the tube bundle are subject to expansion and contraction due to changes in temperature during the heat exchange process. As a result of the vibrations and the temperature changes there is great stress placed upon the tubes at locations where they are rigidly attached. This stress can result in leaks at the points of contact. Locating and repairing those leaks can be a difficult task.
One solution to this problem is to use thicker wall copper tube. This of course adds weight and expense. Another possible solution is to use softer tube support material to absorb movement due to vibration and expansion and contraction. This approach also has its shortcomings. Yet another approach is to allow the tubes to move within support plates while still having the plates provide the direct support for the tubes. This of course produces wear on the tubes, which again results in leaks.
While there are advantages to have each tube share the support function because of the total contact area involved, yet another approach that has been developed utilizes additional support tubes or rods that are attached to support plates, while the fluid-carrying tubes extend through oversized holes in the support plates so that there is little or no contact between the fluid-carrying tubes and the support plates. This is sometimes referred to as a floating tube bundle in the sense that the fluid-carrying tubes can move freely within the support plates. Examples of this system are disclosed in U.S. Pat. No. 5,020,587 and in European patent 0209107. In one instance a relatively large number of copper tubes are employed for the support function. In another instance smaller numbers of tubes or rods are employed using materials stronger than copper. Because of the shortcomings with both approaches, a need exists for an improved support arrangement.
Another source of leaks in heat exchange or tube bundles occurs in the area of the discharge header. Each tubular circuit within a tube bundle requires an input and output connection to a header which extends generally perpendicular to the straight sections of the tube bundle. Bundles with a large number of tubes will have a number of fluid circuits and hence a number of corresponding connections to the header. Leaks can occur if the tube support plates make contact with the tubes that are attached to the header. Thermal expansion or contraction of the tubes causes wear. Most manufacturers have solved this problem by having clearance holes in the tube plate at these locations. Leaks can also be caused by the thermal expansion or contraction of the header itself. Because the expansion coefficient is linear, a long header expands more than a short header. The outermost tubes connected to the header will therefore bend the most. Headers over four feet long can cause fatiguing of the outermost tubes. Most manufacturers now limit the length of the header. In actual application, field manifolding should be configured so that some of the expansion or contraction can be absorbed at the manifolds.
Leaks can also result from improper support of the field piping connected to the headers. Further, the condenser fans and the compressor produce additional vibrations in the piping. The resulting stress is concentrated at the fluid-carrying tubes that tie into the header, primarily at the point where these tubes pass into the bundle. Small circuits with only one, two or three tubes into the header are particularly susceptible to leaks from this cause. Typically, the connection between the header and a tube in the tube bundle is made by a short connector tube which has one end connected to the header and the other end connected to one of the tube ends protruding through a support plate. Leaks typically occur at the ends of these connector tubes.
In view of the foregoing, a need exists for an improved arrangement for supporting a tube bundle in a manner to minimize leaks in the system.